The present invention relates generally to time control circuits and more specifically to time control circuits where primary and secondary time sources are utilized.
There exist many applications where a critical time base is very important. Any application of devices where multiple devices operate independently in a coordinated manner with each independent device coordinated on the basis of an absolute time requires a very critical time base to be kept. An example of such an application is in decentralized traffic controllers. In a decentralized system, independent traffic controllers are located on individual street corners, and generally are independent from traffic controllers located on adjacent street corners. However, specific timing between street corners may be extremely desirable. That is, a specific offset between the "greens" at the consecutive corners or intersections may be desirable in order to reduce overall traffic delays.
Heretofore, the independent traffic controllers located on adjacent street corners needed to be connected to each other, e.g. by cable, to allow for synchronization. This is due primarily because of a generally poor time base availability. If each individual controller is to be truly independent, then strict control must be provided by a time base. Errors in time base of even three or four seconds can be noticeable for drivers approaching various street corners; therefore, it is of critical need to provide a strict time-controlled output.
U.S. Pat. No. 4,145,617 Lee, et al entitled Control Circuit for Providing Time-Selected Application of A.C. Power, issued Mar. 20, 1979 and assigned to the assignee of the present application uses A.C. power as a primary time source and a crystal oscillator as a backup. Generally, over a long term, the A.C. frequency will be more stable than a crystal oscillator subjected to a street corner environment of temperature fluctuation voltage differentiations due to nearby loads and other factors. Lee provides A.C. synchronization of a crystal oscillator with a D.C. battery as backup power to provide power for the circuitry. The Lee patent is hereby incorporated by reference.
However, where very critical timing relationships must be maintained, strict timing control must be maintained even down to one cycle of the primary time source. For example, if the primary time source is sixty (60) hertz, a loss of one cycle during switchover from primary (A.C.) to secondary (oscillator) time source would result in a time error of 1/60 of a second. Over a period of time, such errors could be cumulative and provide a less than strict time control base. Therefore, there is needed a time-control circuit providing accuracy down to even each cycle of the primary time source.